Hot-formed steel sheet member

ABSTRACT

A hot-formed steel sheet member having a chemical composition includes, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, and a reminder consisting of Fe and impurities, wherein a total volume fraction of martensite, tempered martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less, an average grain size of prior γ grains is 10 μm or less, and a number density of residual carbides which are present is 4×10 3  per mm 2  or less.

TECHNICAL FIELD

The present specification relates to a hot-formed steel sheet memberformed by hot-forming a steel sheet.

BACKGROUND ART

A high-strength steel sheet having high tensile strength has been widelyapplied to the field of an automotive steel sheet in order to achieveboth weight saving for an improvement in fuel consumption and animprovement in collision resistance. However, the high strength causesdeterioration in the press formability of the steel sheet, which makesit difficult to produce products having complicated shapes.

As a result, for example, the high strength of the steel sheetdisadvantageously causes deterioration in ductility, which causesbreaking at a site having a high degree of processing, anddisadvantageously causes deterioration in dimension accuracy or the likebecause of increased spring back and wall warpage. Therefore, a steelsheet having high strength, particularly tensile strength of 780 MPa ormore, is not easily press-formed to a product having a complicatedshape.

Then, recent years, for example, as disclosed in Japanese PatentApplication Laid-Open (JP-A) No. 2002-102980, a hot stamp technique isadopted as a technique for press-forming a material which is hard toform such as a high-strength steel sheet. The hot stamp technique is ahot forming technique for heating and forming a material provided forforming. Since the steel sheet is formed and quenched at the same timein the technique, the steel sheet is soft and has favorable formabilityduring forming, and the formed member after forming can have strengthhigher than that of a steel sheet for cold forming.

Japanese Patent Application Laid-Open (JP-A) No. 2006-213959 discloses asteel member having tensile strength of 980 MPa.

Japanese Patent Application Laid-Open (JP-A) No. 2007-314817 disclosesthat a hot pressed steel sheet member having excellent tensile strengthand toughness is obtained by decreasing a cleanliness level andsegregation degrees of P and S.

SUMMARY OF DISCLOSURE

The metal material described in JP-A No. 2002-102980 has insufficienthardenability during hot press, as a result of which the metal materialhas poor hardness stability. The steel sheets having excellent tensilestrength and toughness are disclosed in JP-A No. 2006-213959 and JP-ANo. 2007-314817, but room for an improvement in local deformationcharacteristics is left.

An objective of embodiments of the specification is to provide ahot-formed steel sheet member having excellent hardness stability andlocal deformability. In many cases, a steel sheet member which ishot-formed is not a flat sheet but a formed body, and is referred to as“a hot-formed steel sheet member” including a case in which thehot-formed steel sheet member is the formed body in the specification.

According to one aspect of the present specification, there is provideda hot-formed steel sheet member having a chemical composition consistingof, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si,from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr,from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0 to 0.50% ofNb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to 1.0% of Mo,from 0 to 1.0% of V, from 0 to 0.005% of Ca, and a remainder consistingof Fe and impurities,

wherein a total volume fraction of martensite, tempered martensite, andbainite is 50% or more, and a volume fraction of ferrite is 3% or less,

an average grain size of prior γ grains is 10 μm or less, and

a number density of residual carbides which are present is 4×10³ per mm²or less.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a shape of a mold in hat forming inExamples.

FIG. 2 is a schematic view showing a shape of a formed body obtained byhot-forming in Examples.

FIG. 3 is a schematic view showing a shape of a notch tensile test piecein Examples.

DESCRIPTION OF EMBODIMENTS

The present inventors have conducted studies earnestly to provide ahot-formed steel sheet member having excellent hardness stability andlocal deformability, and resultantly obtained the findings describedbelow.

(1) Fine prior γ grains in the hot-formed steel sheet member delay theoccurrence and connection of voids, which provides an improvement inlocal deformability. Therefore, the fine prior γ grains are preferable.

(2) In a case in which a number of residual carbides are present in thehot-formed steel sheet member, hardenability after hot-forming may bedeteriorated to cause deterioration in hardness stability, and theresidual carbides serve as the occurrence source of voids to causedeterioration in the local deformability. Therefore, the number densityof the residual carbides is preferably reduced.

Embodiments of the specification is based on the findings. According toone aspect of the embodiments,

(1) there is provided a hot-formed steel sheet member having a chemicalcomposition, consisting of, in terms of mass %, from 0.08 to 0.16% of C,0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01%or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B,from 0 to 0.50% of Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from0 to 1.0% of Mo, from 0 to 1.0% of V, from 0 to 0.005% of Ca, and aremainder consisting of Fe and impurities,

wherein a total volume fraction of martensite, tempered martensite, andbainite is 50% or more, and a volume fraction of ferrite is 3% or less,

an average grain size of prior γ grains is 10 μm or less, and

a number density of residual carbides which are present is 4×10³ per mm²or less.

(2) In the hot-formed steel sheet member of (1), the chemicalcomposition preferably includes one or more selected from the groupconsisting of, in terms of mass %, from 0.003 to 0.50% of Nb, from 0.01to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01 to 1.0% of Mo, from0.01 to 1.0% of V, and from 0.001 to 0.005% of Ca.

(3) In the hot-formed steel sheet member of (1) or (2), a value of acleanliness level of steel specified by JIS G 0555 (2003) is preferably0.08% or less.

(4) In any one of the hot-formed steel sheet members of (1) to (3), asegregation degree α of Mn represented by the following formula (i) ispreferably 1.6 or less,

α=[maximum Mn concentration (mass %) at a central part of a sheetthickness]/[average Mn concentration (mass %) at a ¼ depth position ofthe sheet thickness from a surface]  (i).

(5) In any one of the hot-formed steel sheet members of (1) to (4), thesteel sheet member preferably has a surface on which a plating layer isformed.

(6) In any one of the hot-formed steel sheet members of (1) to (5), thesteel sheet member preferably has a tensile strength of 1.0 GPa or more.

Hereinafter, the embodiments will be described in detail.

(A) Chemical Composition

The reason why the content of each element is limited will be describedbelow. In the description below, the symbol “%” of the content of eachelement means “mass %”.

C: from 0.08 to 0.16%

C is an element important for improving the hardenability of steel andfor securing the strength after quenching. Since C is anaustenite-forming element, it has a function to suppress strain-inducedferrite transformation during high strain formation. This makes it easyto obtain a stable hardness distribution in a steel sheet member afterhot-forming. The C content of less than 0.08% makes it difficult tosecure tensile strength of 1.0 GPa or more after quenching and to obtainthe above-mentioned effect. Therefore, the C content is set to 0.08% ormore. The C content exceeding 0.16% causes an excessive increase in thestrength after quenching to cause deterioration in local deformability.Therefore, the C content is set to 0.16% or less. The C content ispreferably 0.085% or more, and more preferably 0.9% or more. The Ccontent is preferably 0.15% or less, and more preferably 0.14% or less.

Si: 0.19% or less

Si is an element having a function to suppress scale formation duringhigh temperature heating for hot-forming. However, the Si contentexceeding 0.19% causes a remarkable increase in a heating temperaturerequired for austenite transformation during hot-forming. This causes anincrease in cost required for a heat treatment, and insufficientquenching due to insufficient heating. Si is a ferrite-forming element.Thereby, a too high Si content is apt to produce strain-induced ferritetransformation during high strain formation. This causes a localdecrease in the hardness of the steel sheet member after hot-forming,which makes it difficult to obtain a stable hardness distribution.Furthermore, a significant amount of Si causes deterioration inwettability in a case in which a hot-dip plating treatment is performed,which may cause non-plating. Therefore, the Si content is set to 0.19%or less. The Si content is preferably 0.15% or less. In a case in whichthe above-mentioned effect is desired to be obtained, the Si content ispreferably 0.01% or more.

Mn: from 0.40 to 1.50%

Mn is an element useful for improving the hardenability of a steel sheetand for stably securing the strength after hot-forming. The Mn contentof less than 0.40% makes it difficult to obtain the above-mentionedeffect. Therefore, the Mn content is set to 0.40% or more. The Mncontent exceeding 1.50% produces coarse MnS, which becomes a factor fordeterioration in local deformability. Therefore, the Mn content is setto 1.50% or less. The Mn content is preferably 0.80% or more, andpreferably 1.40% or less.

P: 0.02% or less

Since P is an element contained as impurities, and has functions to makeit possible to improve the hardenability of steel and to stably securethe strength of the steel after quenching, P may be positivelycontained. However, the P content exceeding 0.02% causes remarkabledeterioration in local deformability. Therefore, the P content is set to0.02% or less. The P content is preferably 0.01% or less. Although thelower limit of the P content is not particularly limited, an excessivereduction in the P content causes a remarkable increase in cost. Forthis reason, the P content is preferably set to 0.0002% or more.

S: 0.01% or less

S is an element contained as impurities, and causing deterioration inlocal deformability. The S content exceeding 0.01% causes remarkabledeterioration in the local deformability. Therefore, the S content isset to 0.01% or less. Although the lower limit of the S content is notparticularly limited, an excessive reduction in the S content causes aremarkable increase in cost. Therefore, the S content is preferably setto 0.0002% or more.

sol. Al: from 0.01 to 1.0%

sol. Al is an element having a function to enable soundness of steel bydeoxidizing molten steel. The sol. Al content of less than 0.01% causesinsufficient deoxidation. Furthermore, since the sol. Al is also anelement having functions to improve the hardenability of a steel sheetand to stably secure the strength after quenching, the sol. Al may bepositively contained. Therefore, the sol. Al content is set to 0.01% ormore. However, the sol. Al content exceeding 1.0% provides a smalleffect obtained by the function, and unnecessarily causes an increase incost. For this reason, the sol. Al content is set to 1.0% or less. Thesol. Al content is preferably 0.02% or more and preferably 0.2% or less.

N: 0.01% or less

N is an element contained as impurities, and causing deterioration intoughness. The N content exceeding 0.01% forms coarse nitride in steel,which causes remarkable deteriorations in local deformability andtoughness. Therefore, the N content is set to 0.01% or less. The Ncontent is preferably 0.008% or less. Although the lower limit of the Ncontent need not be particularly limited, an excessive reduction in theN content causes a remarkable increase in cost. For this reason, the Ncontent is preferably set to 0.0002% or more, and more preferably0.0008% or more.

Cr: from 0.25 to 3.00%

Cr is an element having a function to improve the hardenability ofsteel. Therefore, Cr is a particularly important element in anembodiment in which the Mn content is limited to 1.50% or less. Cr is anaustenite-forming element, and has a function to suppress strain-inducedferrite transformation during high strain formation. Therefore, Cr iscontained, which makes it easy to obtain a stable hardness distributionin a steel sheet member after hot-forming. The Cr content of less than0.25% cannot sufficiently provide the above-mentioned effect. Therefore,the Cr content is set to 0.25% or more. The Cr content exceeding 3.00%causes Cr to be incrassated in carbonates in carbonates in the steel,which delays the solid solution of the carbides in a heating step in thecase of being provided for hot-forming to cause deterioration in thehardenability. Therefore, the Cr content is set to 3.00% or less. The Crcontent is preferably 0.30% or more, and more preferably 0.40% or more.The Cr content is preferably 2.50% or less, and more preferably 2.00% orless.

Ti: from 0.01 to 0.05%

Ti is an element having a function to suppress the recrystallization ofaustenite grains in a case in which a steel sheet for hot-forming isheated to an Ac₃ point or more and provided for hot-forming.Furthermore, Ti has a function to form fine carbides to suppress thegrain growth of the austenite grains, thereby providing fine grains. Forthis reason, Ti has a function to largely improve the localdeformability of a hot-formed steel sheet member. Since Ti ispreferentially bonded to N in steel, Ti suppresses the consumption of Bdue to the precipitation of BN, as a result of which Ti has a functionto improve hardenability due to B. Therefore, the Ti content is set to0.01% or more. However, the Ti content exceeding 0.05% causes anincrease in the amount of precipitation of TiC, which causes theconsumption of C, thereby causing a decrease in the strength afterquenching. For this reason, the Ti content is set to 0.05% or less. TheTi content is preferably 0.015% or more. The Ti content is preferably0.04% or less, and more preferably 0.03% or less.

B: from 0.001 to 0.01%

B is an element having functions to makes it possible to improve thehardenability of steel and to stably secure the strength afterquenching. Therefore, in an embodiment in which the Mn content islimited to 1.50% or less, B is a particularly important element. The Bcontent of less than 0.001% cannot sufficiently provide theabove-mentioned effect. Therefore, the B content is set to 0.001% ormore. The B content exceeding 0.01% causes the saturation of theabove-mentioned effect, and deterioration in the local deformability ofa quenched part. Therefore, the B content is set to 0.01% or less. The Bcontent is preferably 0.005% or less.

The hot-formed steel sheet members of the embodiments have a chemicalcomposition consisting of the elements of C to B and the remainderconsisting of Fe and impurities.

The “impurities” herein are elements which are mixed in by variousfactors in raw materials such as ore or scrap and in a productionprocess when a steel sheet is produced on an industrial scale, and areallowed to be contained within the range such that the elements do notexert an adverse influence on the embodiments.

The hot-formed steel sheet member of the embodiments may further containone or more elements selected from the group consisting of Nb, Ni, Cu,Mo, V, and Ca in amounts to be described below in addition to theabove-mentioned elements.

Nb: from 0 to 0.50%

Nb is an element having functions to suppress recrystallization in acase in which a steel sheet for hot-forming is heated to an Ac₃ point ormore and provided for hot-forming, and to form fine carbides to suppressthe grain growth, thereby providing fine austenite grains. For thisreason, Nb has a function to largely improve the local deformability ofa hot-formed steel sheet member. Therefore, Nb may be contained ifnecessary. However, the Nb content exceeding 0.50% causes an increase inthe amount of precipitation of NbC to cause the consumption of C,thereby causing a decrease in the strength after quenching. For thisreason, the Nb content is set to 0.50% or less. The Nb content ispreferably 0.45% or less. In a case in which the above-mentioned effectis desired to be obtained, the Nb content is preferably set to 0.003% ormore, and more preferably 0.005% or more.

Ni: from 0 to 2.0%

Since Ni is an element effective in improving the hardenability of steelsheet and in stably securing the strength after quenching, Ni may becontained if necessary. However, the Ni content exceeding 2.0% providesa small effect, which unnecessarily causes an increase in cost. For thisreason, the Ni content is set to 2.0% or less. The Ni content ispreferably 1.5% or less. In a case in which the above-mentioned effectis desired to be obtained, the Ni content is preferably set to 0.01% ormore, and more preferably 0.05% or more.

Cu: from 0 to 1.0%

Since Cu is an element effective in improving the hardenability of steelsheet and in stably securing the strength after quenching, Cu may becontained if necessary. However, the Cu content exceeding 1.0% providesa small effect, which unnecessarily causes an increase in cost. For thisreason, the Cu content is set to 1.0% or less. The Cu content ispreferably 0.5% or less. In a case in which the above-mentioned effectis desired to be obtained, the Cu content is preferably set to 0.01% ormore, and more preferably 0.03% or more.

Mo: from 0 to 1.0%

Mo is an element having a function to form fine carbides in a case inwhich a steel sheet for hot-forming is heated to an Ac₃ point or moreand provided for hot-forming to suppress the grain growth, therebyproviding fine austenite grains. Mo has also an effect of largelyimproving the local deformability of a hot-formed steel sheet member.For these reasons, Mo may be contained if necessary. However, the Mocontent exceeding 1.0% causes the saturation of the effect, whichunnecessarily causes an increase in cost. Therefore, the Mo content isset to 1.0% or less. The Mo content is preferably 0.7% or less. In acase in which the above-mentioned effect is desired to be obtained, theMo content is preferably set to 0.01% or more, and more preferably 0.04%or more.

V: from 0 to 1.0%

Since V is an element effective in improving the hardenability of steelsheet and in stably securing the strength after quenching, V may becontained if necessary. However, the V content exceeding 1.0% provides asmall effect, which unnecessarily causes an increase in cost. For thisreason, the V content is set to 1.0% or less. The V content ispreferably 0.08% or less. In a case in which the effect is desired to beobtained, the V content is preferably set to 0.01% or more, and morepreferably 0.02% or more.

Ca: from 0 to 0.005%

Since Ca is an element having an effect of grain refining of inclusionsin steel to improve the local deformability after quenching, Ca may becontained if necessary. However, the Ca content exceeding 0.005% causesthe saturation of the effect, which unnecessarily causes an increase incost. Therefore, the Ca content is set to 0.005% or less. The Ca contentis preferably 0.004% or less. In a case in which the effect is desiredto be obtained, the Ca content is preferably set to 0.001% or more, andmore preferably 0.002% or more.

(B) Metal Structure

In the embodiments, in order to improve local deformability, variationsin hardness in the metal structure after hot-forming is preferablysuppressed. Since an increased hardness difference in the structureserves as the starting point of voids, the mixture of a low-temperaturetransformation structure such as hard martensite or bainite and a softferrite structure is preferably suppressed as much as possible.Therefore, it is preferable that the hot-formed steel sheet members ofthe embodiments mainly have a low-temperature transformation structure,and has a metal structure having a ferrite volume fraction of 3% orless.

The metal structure mainly having a low-temperature transformationstructure means a metal structure in which the total volume fraction ofmartensite, tempered martensite, and bainite is 50% or more. Thetempered martensite herein means martensite transformed during quenchingand tempered by automatic tempering, and martensite subjected to lowtemperature tempering such as a coating baking process after quenching.The volume fraction of the low-temperature transformed structure in themetal structure is preferably 80% or more, and more preferably 90% ormore.

Since residual austenite improves ductility according to the TRIPeffect, the residual austenite is uneventfully contained. However,martensite transformed from austenite is hard, which serves as thestarting point of voids. Therefore, the volume fraction of the residualaustenite contained in the metal structure is preferably 10% or less.

Segregation Degree α of Mn: 1.6 or less

α=[maximum Mn concentration (mass %) at a central part of a sheetthickness]/[average Mn concentration (mass %) at ¼ depth position ofsheet thickness from a surface]  (i)

At the central part of the section of the sheet thickness of thehot-formed steel sheet member, center segregation occurs, whichincrassates Mn. Therefore, MnS concentrates on the center as inclusions,which is apt to cause the formation of hard martensite. This causes adifference in hardness between the hard martensite and itscircumference, as a result of which the local deformability isdeteriorated. Particularly, in a case in which the value of thesegregation degree α of Mn represented by the formula (i) exceeds 1.6,the local deformability is remarkably deteriorated. Therefore, in orderto improve the local deformability, the a value of the hot-formed steelsheet member is preferably set to 1.6 or less. In order to furtherimprove the local deformability, the a value is more preferably set to1.2 or less.

The segregation of Mn in the steel sheet is mainly controlled by a steelsheet composition (particularly an impurity content) and a continuouscasting condition, and is not substantially changed after and beforehot-rolling and hot-forming. Therefore, the inclusions and segregationsituation of the steel sheet for hot-forming are almost the same asthose of the hot-formed steel sheet member manufactured by hot-formingthe steel sheet for hot-forming. Since the a value is not largelychanged by hot-forming, the a value of the hot-formed steel sheet membercan also be set to 1.6 or less by setting the a value of the steel sheetfor hot-forming to 1.6 or less. The a value of the hot-formed steelsheet member can also be set to 1.2 or less by setting the a value to1.2 or less.

The maximum Mn concentration at a central part of the sheet thickness isobtained by the following method. The central part of the sheetthickness of the steel sheet is subjected to line analysis using anelectron probe microanalyzer (EPMA). Three measured values are selectedin higher order from the analysis results, and the average value thereofis calculated. The average Mn concentration at the ¼ depth position ofthe sheet thickness from the surface is obtained by the followingmethod. Similarly, ten places are analyzed at the ¼ depth position ofthe steel sheet using EPMA, and the average value thereof is calculated.

Cleanliness Level: 0.08% or less

In a case in which A-based, B-based, and C-based inclusions described inJIS G 0555 (2003) exist in large amounts in the steel sheet member, theinclusions are apt to serve as the starting point of breaking. In a casein which the inclusions are increased, crack propagation easily occurs,which causes deterioration in the local deformability. Particularly, inthe case of the hot-formed steel sheet member having tensile strength of1.0 GPa or more, the existence fraction of the inclusions is preferablysuppressed low. In a case in which the value of the cleanliness level ofthe steel specified by JIS G 0555 (2003) exceeds 0.08%, the amount ofthe inclusions is large, which makes it difficult to secure practicallysufficient local deformability. Therefore, the value of the cleanlinesslevel of the steel sheet for hot-forming is preferably set to 0.08% orless. The value of the cleanliness level is more preferably set to 0.04%or less in order to further improve the local deformability. The valueof the cleanliness level of the steel is obtained by calculating thearea percentages of the A-based, B-based, and C -based inclusions.

Since the value of the cleanliness level is not largely changed byhot-forming, the value of the cleanliness level of the hot-formed steelsheet member can also be set to 0.08% or less by setting the value ofthe cleanliness level of the steel sheet for hot-forming to 0.08% orless. The value of the cleanliness level of the hot-formed steel sheetmember can also be set to 0.04% or less by setting the value of thecleanliness level of the steel sheet for hot-forming to 0.04% or less.

In the embodiments, the value of the cleanliness level of the steelsheet for hot-forming or the hot-formed steel sheet member is obtainedby the following method. Test materials are cut from five places of thesteel sheet for hot-forming or the hot-formed steel sheet member. In acase in which the sheet thickness of the steel sheet for hot-forming orthe hot-formed steel sheet member is defined as t, the cleanliness levelis investigated at each of positions of ⅛t, ¼t, ½t, ¾t, and ⅞t in thedirection of the sheet thickness of each of the test materials by aJIS-G-0555 method. The largest value (lowest cleanliness property) ofthe cleanliness level in each of the sheet thicknesses is used as thevalue of the cleanliness level of the test material.

Average Grain Size of Prior γ Grains: 10 μm or less

In a case in which a prior γ grain size in the hot-formed steel sheetmember is decreased, the local deformability is improved. In a steelsheet mainly containing martensite, voids occur at prior γ grainboundaries and boundaries of the lower structures in grains. However,grain refining of prior γ grains can suppress the occurrence of thevoids, and improve the local deformability for delaying connection. In acase in which the average grain size of the prior γ exceeds 10 μm, thiseffect cannot be exhibited. Therefore, the average grain size of theprior γ grains in the hot-formed steel sheet member is set to 10 μm orless. In order to perform grain refining of the prior γ grains, it iseffective to decrease a heating temperature, and to delay thedissolution of carbides during heating to suppress the grain growth.

The average grain size of the prior γ grains can be measured using amethod specified by IS0643. That is, the number of crystal grains in ameasured view is measured. The average area of the crystal grains isobtained by dividing the area of the measured view by the number of thecrystal grains, and the crystal grain size in an equivalent circulardiameter is calculated. At that time, it is preferable that the grain onthe boundary of the view is measured as ½, and a magnification ratio isadjusted so that the number of the crystal grains is set to 200 or more.A plurality of views are preferably measured in order to improveaccuracy.

Residual Carbides: 4×10³ per mm² or less

In the case of hot-forming, sufficient hardenability can be secured bythe resolution of carbides generally existing in steel. However, a partof the carbides may remain without being resolved. The residual carbideshave an effect of suppressing the growth of γ grains in holding heatingduring hot-forming by pinning. Therefore, the residual carbidesdesirably exist during holding heating. As the residual carbides aredecreased after hot-forming, the hardenability is improved, which canprovide the securement of high strength. Therefore, it is preferablethat the number density of the residual carbides can be reduced in acase in which the holding heating is completed.

In a case in which a number of residual carbides exist, thehardenability after hot-forming may be deteriorated, and the residualcarbides serve as the occurrence source of voids to cause deteriorationin local deformability. Particularly, in a case in which the numberdensity of the residual carbides exceeds 4×10³ per mm², thehardenability after hot-forming may be deteriorated. Therefore, thenumber density of the residual carbides existing in the hot-formed steelsheet member is preferably 4×10³ per mm² or less.

(C) Plating Layer

The high-strength hot-formed steel sheet member according to theembodiments may have a surface on which a plating layer is formed forthe purpose of an improvement in corrosion resistance, or the like. Theplating layer may be an electroplating layer, and may be a hot-dipplating layer. Examples of the electroplating layer includeelectrogalvanizing, electric Zn—Ni alloy plating, and electric Zn—Fealloy plating. Examples of the hot-dip plating layer include hot dipgalvanizing, alloyed hot dip galvanizing, molten aluminum plating,molten Zn—Al alloy plating, molten Zn—Al—Mg alloy plating, and moltenZn—Al—Mg—Si alloy plating. A plating deposition amount is notparticularly limited, and may be adjusted within a general range.

(D) Method for Manufacturing Steel Sheet for Hot-Forming

The manufacturing conditions of the steel sheet for hot-forming used formanufacturing the steel sheet member for hot-forming according to theembodiments are not particularly limited, but the steel sheet forhot-forming can be suitably manufactured by using a manufacturing methodto be shown below.

The steel having the above-mentioned chemical composition is melted in afurnace, and a slab is then produced by casting. In order to set thecleanliness level of the steel sheet to 0.08% or less, it is desirableto set the heating temperature of molten steel to a temperature higherby 5° C. or more than the liquidus-line temperature of the steel in acase in which the molten steel is continuously cast, and to suppress theamount of the molten steel to be cast per unit time to 6 t/min or less.

In a case in which the amount to be cast per unit time of the moltensteel exceeds 6 t/min during continuous casting, the molten steel isfast stirred in a mold. Thereby, inclusions are apt to be trapped by asolidifying shell, which causes an increase in the inclusions in theslab. In a case in which the molten steel heating temperature is lessthan a temperature higher by 5° C. than the liquidus-line temperature,the viscosity of the molten steel is increased, and thereby, theinclusions are less likely to float in a continuous-casting machine. Asa result, the inclusions in the slab are increased, which is apt tocause deterioration in cleanliness property.

The molten steel is cast with the molten steel heating temperature setto 5° C. or higher from the liquidus-line temperature of the moltensteel and the amount of the molten steel to be cast per unit time set to6 t/min or less, which is less likely to cause the introduction of theinclusions into the slab. As a result, the amount of the inclusions atthe stage in which the slab is produced can be effectively decreased,which can easily achieve the steel sheet cleanliness level of 0.08% orless.

In a case in which the molten steel is continuously cast, the moltensteel heating temperature is more desirably set to a temperature higherby 8° C. or more than the liquidus-line temperature, and the amount ofthe molten steel to be cast per unit time is more desirably set to 5t/min or less. By setting the molten steel heating temperature to atemperature higher by 8° C. or more than the liquidus-line temperature,and setting the amount of the molten steel to be cast per unit time to 5t/min or less, the cleanliness level is easily set to 0.04% or less,which is desirable.

In order to suppress the concentration of MnS causing deterioration inlocal deformability, a center segregation reducing treatment isdesirably performed to reduce the center segregation of Mn. Examples ofthe center segregation reducing treatment include a method ofdischarging molten steel in which Mn is incrassated in an unsolidifiedlayer before a slab is completely solidified.

Specifically, molten steel in which Mn before being completelysolidified is incrassated can be discharged by a treatment such aselectromagnetic stirring or unsolidified layer reduction. Theelectromagnetic stirring treatment can be performed by stirringunsolidified molten steel at from 250 to 1000 gausses, for example. Theunsolidified layer reduction treatment can be performed by reducing alast solidified part at the slope of about 1 mm/m, for example.

The slab obtained by the above-mentioned method may be subjected to asoaking treatment if necessary. By performing the soaking treatment,segregated Mn is diffused, which can provide a reduction in asegregation degree. A preferable soaking temperature in a case in whichthe soaking treatment is performed is from 1200 to 1300° C., and apreferable soaking time is from 20 to 50 hours.

Then, the slab is hot-rolled. As hot-rolling conditions, from theviewpoint of more uniformly producing carbides, it is preferable that ahot-rolling initiation temperature is set to a temperature region offrom 1000 to 1300° C., and a hot-rolling completion temperature is setto 850° C. or higher. A winding temperature is preferably higher fromthe viewpoint of processability. However, in a case in which the windingtemperature is too high, scale formation causes a decrease in yield, andthereby the winding temperature is preferably from 500 to 650° C. Ahot-rolled steel sheet obtained by hot-rolling is subjected to a descaletreatment by pickling or the like.

In the embodiments, in order to perform grain refining of prior γ grainsafter hot-forming and to reduce the number density of residual carbides,the hot-rolled steel sheet subjected to the descale treatment ispreferably annealed to produce a hot-rolled annealed steel sheet.

In order to provide the fine prior γ grain size after hot-forming, thegrowth of the γ grains is preferably suppressed by the carbides insolution. However, in order to improve the hardenability, to secure thehigh strength, and to suppress the occurrence of voids in the hot-formedsteel sheet member, the number density of the residual carbides ispreferably reduced.

In order to provide the fine prior γ grain size in the hot-formed steelsheet member and to reduce the number density of the residual carbides,the form of the carbides existing in the steel sheet before hot-formingand the incrassating degree of elements in the carbides are important.It is desirable that the carbides are finely dispersed. However, sincethe carbides are fast dissolved in the case, a grain growth suppressingeffect cannot be expected. In a case in which elements such as Mn and Crare incrassated in the carbides, the carbides are less likely to besolved. Therefore, it is desirable that the carbides in the steel sheetbefore hot-forming are finely dispersed, and the incrassating degree ofthe elements in the carbides is higher.

The form of the carbides can be controlled by adjusting the annealingcondition after hot-rolling. Specifically, it is preferable that theannealing temperature is set to an Ac1 point or less and the Ac1point-100° C. or higher, and an annealing time is 5 hours or less.

In a case in which a winding temperature after hot-rolling is set to550° C. or lower, the carbides are likely to be finely dispersed.However, since the incrassating degree of the elements in the carbidesis also decreased, the incrassating of the elements is advanced byannealing.

In a case in which the winding temperature is 550° C. or higher, perliteis generated, and the incrassating of the elements to the carbides inthe perlite is advanced. In this case, in order to divide the perlite todisperse the carbides, annealing is performed.

As the steel sheet for hot-formed steel sheet member in the embodiments,the above-mentioned hot-rolled annealed steel sheet, a cold-rolled steelsheet obtained by cold-rolling the hot-rolled annealed steel sheet, or acold-rolled annealed steel sheet obtained by annealing the cold-rolledsteel sheet can be used. A treating step may be selected if appropriateaccording to the request level of the accuracy of the sheet thickness ofa product, or the like. Since the carbides are hard, the form of thecarbides is not changed even in a case in which cold-rolling isperformed, and the existence form before cold-rolling is maintained evenafter cold-rolling.

The cold-rolling may be performed using a usual method. From theviewpoint of securing favorable flatness, a reduction ratio in thecold-rolling is preferably set to 30% or more. In order to avoid anexcessive load, the reduction ratio in the cold-rolling is preferablyset to 80% or less.

In a case in which the cold-rolled steel sheet is annealed, it isdesirable that the cold-rolled steel sheet is preliminarily subjected toa treatment such as degreasing. The annealing is preferably performed atan Ac1 point or less, for hours or less, preferably for 3 hours or lessfor the purpose of cold-rolling strain lessening.

(E) Method for Forming Plating Layer

As described above, the hot-formed steel sheet member according to theembodiments may have a surface on which a plating layer is formed forthe purpose of an improvement in corrosion resistance, or the like. Theplating layer is desirably formed on the steel sheet before beingsubjected to hot-forming. In a case in which zinc-based plating isapplied to the surface of the steel sheet, molten zinc-based plating ispreferably applied in a continuous hot dip galvanizing line from theviewpoint of productivity. In the case, annealing may be performedbefore a plating treatment in the continuous hot dip galvanizing line.Only a plating treatment may be performed without being annealed with aheat holding temperature set to a low temperature. An alloyed moltenzinc sheeted sheet steel may be provided by performing an alloying heattreatment after hot dip galvanizing. The zinc-based plating can also beapplied by electroplating. The zinc-based plating can be applied to atleast a part of the surface of the steel material. However, generally,the zinc-based plating is entirely applied to one surface or bothsurfaces of the steel sheet.

(F) Method for Manufacturing Hot-Formed Steel Sheet Member

By hot-forming the steel sheet for hot-forming, a high-strengthhot-formed steel sheet member can be obtained. From the viewpoint ofsuppressing the grain growth, the heating rate of the steel sheet duringhot-forming is desirably 20° C./sec or higher, and more preferably 50°C./sec or higher. The heating temperature of the steel sheet duringhot-forming is desirably set to a temperature of more than an Ac₃ pointand 1050° C. or lower. In a case in which the heating temperature is theAc₃ point or less, ferrite, perlite, or bainite remains in the steelsheet without providing an austenite single phase state beforehot-forming. As a result, desired hardness may not be obtained withoutproviding the metal structure mainly containing martensite afterhot-forming. This causes not only an increase in a variation in hardnessof the hot-formed steel sheet member but also deterioration in localdeformability.

In a case in which the heating temperature exceeds 1050° C., theaustenite is coarse, which may cause deterioration in the localdeformability of the steel sheet member. Therefore, the heatingtemperature of the steel sheet during hot-forming is preferably set to1050° C. or lower. In a case in which a heating time is less than 1 min,the single-phasing of the austenite may be insufficient even if heatingis performed. Furthermore, since the dissolution of the carbides isinsufficient, the number density of the residual carbides is increasedeven if the γ grain size is refined. In a case in which the heating timeexceeds 10 min, the austenite is coarse, which may cause deteriorationin the local deformability of the hot-formed steel sheet member.Therefore, the heating time of the steel sheet during hot-forming isdesirably set to from 1 to 10 min.

In a case in which a hot-forming initiation temperature is less than theAr₃ point, ferrite transformation starts. Therefore, even if forciblecooling is then performed, the structure mainly containing martensitemay not be provided. Therefore, the hot-forming initiation temperatureis desirably the Ar₃ point or more. Rapid cooling is desirably performedat the cooling rate of 10° C./sec or higher after hot-forming, and rapidcooling is more desirably performed at the rate of 20° C./sec or higher.The upper limit of the cooling rate is not particularly specified.

In order to obtain a hot-formed steel sheet member having a metalstructure mainly containing martensite having a less variation inhardness, the steel sheet after hot-forming is desirably rapidly cooleduntil the surface temperature of the steel sheet becomes 350° C. orlower. A cooling end temperature is preferably set to 100° C. or lower,and more preferably room temperature.

Hereinafter, the embodiments will be more specifically described withreference to Examples, but the present invention is not limited to theseExamples.

EXAMPLES

Steel having chemical components shown in Table 1 was melted in a testconverter, and subjected to continuous casting in a continuous castingtesting machine, to produce slabs each having a width of 1000 mm and athickness of 250 mm. Symbol * used in Table 1 means departing from thecomposition range of the embodiments. Under conditions shown in Table 2,the heating temperature of molten steel and the amount of molten steelto be cast per unit time were adjusted. The cooling rate of each of theslabs was controlled while the water amount of a secondary cooling sprayband was changed. A center segregation reducing treatment is performedby carrying out soft reduction at the slope of 1 mm/m using rolls in asolidified terminal part and discharging the incrassated molten steel ofa last solidified part. A part of the slabs were then subjected to asoaking treatment under conditions of 1250° C. and 24 hours.

TABLE 1 molten steel line steel chemical composition (mass %, balance:Fe and impurities) temperature type C Si Mn P S sol. Al N Cr Ti B Nb CuNi Mo V Ca (° C.) A 0.11 0.15 1.25 0.004 0.002 0.04 0.0015 0.48 0.0180.0015 — — — — — — 1520 B 0.14 0.10 1.00 0.005 0.002 0.03 0.0020 0.700.020 0.0016 — — — — — — 1518 C 0.09 0.05 1.10 0.003 0.002 0.05 0.00241.00 0.023 0.0018 0.08 — — — — — 1523 D 0.15 0.15 1.20 0.004 0.002 0.050.0020 0.48 0.022 0.0030 — 0.1 — — — — 1518 E 0.13 0.05 1.30 0.005 0.0020.02 0.0030 0.60 0.025 0.0022 — — 0.3 — — — 1519 F 0.11 0.10 1.05 0.0040.002 0.03 0.0012 0.70 0.020 0.0020 — — — 0.1 — — 1522 G 0.12 0.02 1.400.005 0.002 0.04 0.0020 1.30 0.018 0.0019 — — — — 0.01 — 1517 H 0.130.05 1.30 0.004 0.003 0.03 0.0022 0.80 0.022 0.0022 — — — — — 0.003 1519I 0.15 0.05 1.30 0.003 0.012* 0.04 0.0023 1.00 0.020 0.0015 — — — — — —1517 J 0.11 0.10 2.40* 0.005 0.002 0.05 0.0025 0.30 0.015 0.0020 — — — —— — 1515 K 0.14 1.00* 1.30 0.004 0.002 0.03 0.0020 0.30 0.019 0.0018 — —— — — — 1508 L 0.20* 0.15 1.30 0.006 0.002 0.04 0.0015 0.40 0.022 0.0015— — — — — — 1513 M 0.11 0.15 0.80 0.005 0.002 0.04 0.0025 0.20* 0.0210.0015 — — — — — — 1523

TABLE 2 amount of molten steel molten slab center annealing afterheating steel to segregation winding hot-rolling test steel temperaturebe cast reducing soaking temperature temperature time number type steelsheet (° C.) (t/min) treatment treatment (° C.) (° C.) (h) 1 Ahot-rolling 1550 4.3 YES 1250° C. × 24 h 510 620 1 2 A cold-rolling 15508 YES 1250° C. × 24 h 510 620 1 3 A cold-rolling 1550 4.3 NO NO 510 6201 4 B hot-rolling 1550 3.5 YES 1250° C. × 24 h 510 620 1 5 Bcold-rolling 1520 5.5 YES NO 510 620 1 6 B cold-rolling 1550 3.5 YES1250° C. × 24 h 510 620 1 7 C cold-rolling 1550 5.1 YES 1250° C. × 24 h510 620 1 8 D cold-rolling 1550 5.5 YES 1250° C. × 24 h 510 620 1 9 Dcold-rolling 1550 5.5 YES 1250° C. × 24 h 510 620 1 10 E cold-rolling1550 3.6 YES 1250° C. × 24 h 510 620 1 11 E cold-rolling 1550 3.6 YES1250° C. × 24 h 680 — — 12 F cold-rolling 1550 2.1 YES 1250° C. × 24 h510 620 1 13 G cold-rolling 1550 5.2 YES 1250° C. × 24 h 510 620 1 14 Gcold-rolling 1550 5.2 YES 1250° C. × 24 h 510 650 20 15 H cold-rolling1550 3.9 YES 1250° C. × 24 h 510 620 1 16 I* hot-rolling 1550 3.9 YES1250° C. × 24 h 510 620 1 17 J* cold-rolling 1550 2.8 NO NO 510 620 1 18K* cold-rolling 1550 2.7 YES 1250° C. × 24 h 510 620 1 19 L*cold-rolling 1550 5.1 YES 1250° C. × 24 h 510 620 1 20 M* cold-rolling1550 4.5 YES 1250° C. × 24 h 510 620 1 metal structure volume fractionhot-forming of annealing holding tensile variation in low-temperaturetest after temperature time strength hardness transformation numbercold-rolling cold-rolling (° C.) (h) (MPa) HS₈₀ HS₁₀ ΔHv structure (%) 1NO NO 880 90 1236 391 371 20 95.6 2 YES YES 880 90 1226 386 363 23 95.43 YES NO 880 90 1230 388 364 24 95.8 4 NO NO 880 90 1345 419 399 20 94.85 YES NO 880 90 1351 422 403 19 94.5 6 YES NO 820 90 1230 420 404 1687.0 7 YES YES 880 90 1169 376 359 16 95.9 8 YES YES 880 90 1384 429 40920 94.6 9 YES YES 1100 90 1356 426 408 18 96.0 10 YES NO 880 90 1305 407387 20 94.6 11 YES NO 880 90 1255 389 350 39 94.2 12 YES NO 880 90 1222384 362 22 94.8 13 YES YES 880 90 1276 402 384 18 94.3 14 YES YES 880 901055 360 265 95 81.0 15 YES YES 880 90 1315 412 394 18 94.7 16 NO NO 88090 1384 429 409 20 94.8 17 YES NO 880 90 1236 391 372 19 94.8 18 YES YES880 90 1343 418 340 77 86.5 19 YES NO 880 90 1550 467 441 26 94.2 20 YESNO 880 90 1242 394 222 172 97.7 number metal structure density volumevolume of fraction fraction residual of of prior γ carbides notch testferrite residual grain cleanliness degree of (per elongation number (%)γ (%) size level (%) segregation α mm²) (%)  1 1.2 3.2 8.8 0.02 1.2 1.0× 10³ 7.9 Examples  2 1.5 3.1 8.3 0.09 1.2 1.5 × 10³ 5.5 ComparativeExamples  3 1.3 2.9 9.0 0.02 1.7 1.8 × 10³ 5.4 Comparative Examples  42.0 3.2 8.6 0.02 1.2 1.6 × 10³ 6.8 Examples  5 2.1 3.4 9.5 0.09 1.3 2.0× 10³ 5.4 Comparative Examples  6 9.6 3.0 5.5 0.03 1.3 8.0 × 10³* 4.5Comparative Examples  7 1.0 3.1 5.4 0.02 1.2 2.2 × 10³ 8.6 Examples  82.0 3.4 8.7 0.02 1.2 1.1 × 10³ 6.5 Examples  9 0.0 4.0 20.1* 0.02 1.20.1 × 10³ 5.4 Comparative Examples 10 1.2 4.2 9.2 0.02 1.1 1.4 × 10³ 7.2Examples 11 3.5 2.3 6.5 0.02 1.1 4.5 × 10³* 5.7 Comparative Examples 121.3 3.9 8.6 0.02 1.2 1.5 × 10³ 7.2 Examples 13 0.5 5.2 8.4 0.02 1.1 2.9× 10³ 7.8 Examples 14 18.1 0.9 5.1 0.02 1.1 8.5 × 10³* 4.8 ComparativeExamples 15 1.3 4.0 9.3 0.03 1.1 1.8 × 10³ 7.3 Examples 16 0.6 4.6 8.80.09 1.1 2.3 × 10³ 5.1 Comparative 17 0.4 4.8 8.4 0.04 1.9 0.8 × 10³ 5.6Examples 18 11.0 2.5 6.5 0.02 1.2 2.9 × 10³ 6.6 19 2.0 3.8 10.2* 0.021.1 1.1 × 10³ 4.6 20 0.5 1.8 8.9 0.02 1.2 0.7 × 10³ 7.8

The obtained slabs were hot-rolled with a hot-rolling testing machine,to produce 3.0-mm-thick hot-rolled steel sheets. Each of the hot-rolledsteel sheets was wound, then subjected to pickling, and furtherannealed. A part of the steel sheets were further cold-rolled with acold-rolling testing machine, to produce 1.5-mm-thick cold-rolled steelsheets. Furthermore, a part of the cold-rolled steel sheets wereannealed at 600° C. for 2 h to obtain cold-rolled annealed steel sheets.

Then, as shown in FIG. 1 and FIG. 2, the steel sheets 1 for hot-formingwere subjected to hot pressing (hat forming) with a mold (punch 11, dice12) using a hot pressing test apparatus, to obtain hot-formed steelsheet members 2. The steel sheets were heated at various surfacetemperatures ranging from 820° C. to 1100° C. in a heating furnace, heldat the temperatures for 90 seconds, then taken out from the heatingfurnace, immediately subjected to hot pressing with the mold with acooling device, and subjected to a quenching treatment simultaneouslywith forming. The hot-formed steel sheet members were evaluated asfollows. The evaluation results are shown in Table 2. In Table 2,“hot-rolling” means a 3.0-mm-thick-hot-rolled steel sheets subjected tohot-rolling, and “cold-rolling” means a 1.5-mm-thick-cold-rolled steelsheet obtained by further cold-rolling the hot-rolled steel sheets.Symbol * means departing from the range of the embodiments.

<Evaluation of Mechanical Characteristics of Hot-Formed Steel SheetMember>

A JIS No. 5 tensile test pieces were obtained from the rollingright-angle direction of the hot-formed steel sheet members, andsubjected to a tensile test according to JIS Z2241 (2011) to measuretensile strength (TS).

<Identification of Metal Structure>

The hot-formed steel sheet members were cut to samples so that thecentral part of the sheet thickness of sections parallel to the rollingdirection, of the hot-formed steel sheet members were viewing surfaces,and the samples were then subjected to mirror polishing. Then, thesamples were subjected to Nital corrosion, and the metal structures offive views of each of the samples were observed using a scanningelectron microscope (magnification ratio: 2000). By subjecting theobtained microphotograph to an image treatment, the area fraction offerrite was obtained. It was used as the volume fraction of ferrite. Thevolume fraction of residual austenite in the metal structure wasobtained using X diffraction (XRD). The balance thereof was calculatedas the volume fraction of a low-temperature transformation structure.The residual γ volume fraction was obtained from the intensity ratio ofdiffraction intensity Iα(200) of (200) of ferrite, diffraction intensityIα(211) of (211) of ferrite, diffraction intensity Iγ(220) of (220) ofaustenite, and diffraction intensity Iγ(311) of (311) of austeniteaccording to X diffraction using a Mo bulb after chemically polishingthe ⅛ inner layer of the sheet thickness from the surface of each of thesteel sheets.

Vγ(volume%)=0.25×{Iγ(220)/(1.35×Iα(200)+Iγ(220))+Iγ(220)/(0.69×Iα(211)+Iγ(220))+Iγ(311)/(1.5×Iα(200)+Iγ(311))+Iγ(311)/(0.69×Iα(211)+Iγ(311))}

<Evaluation of Cleanliness Level>

Test materials were cut from five places of the hot-formed steel sheetmembers. The cleanliness level was investigated at each of positions of⅛t, ¼t, ½t, ¾t, and ⅞t with respect to the sheet thickness t of each ofthe test materials by a point counting method. The largest value (lowestcleanliness property) of the cleanliness level in each of the sheetthicknesses was used as the value of the cleanliness level of the testmaterial.

<Measurement of Segregation Degree α of Mn>

The central part of the sheet thickness of the hot-formed steel sheetmember was subjected to line analysis using EPMA. Three measured valueswere measured at high order from the analysis results, and the averagevalue thereof was then calculated to obtain the maximum Mn concentrationin the central part of the sheet thickness. Ten places were analyzedusing EPMA at the ¼ depth position of the sheet thickness from thesurface of the hot-formed steel sheet member, to obtain the averagevalue thereof. The average Mn concentration at the ¼ depth position ofthe sheet thickness from the surface was obtained. The segregationdegree α of Mn was obtained by dividing the maximum Mn concentration inthe central part of the sheet thickness by the average Mn concentrationat the ¼ depth position of the sheet thickness from the surface.

<Measurement of Average Grain Size of Prior γ Grains>

The average grain size of the prior γ grains in the hot-formed steelsheet member was obtained by measuring the number of crystal grains in ameasured view, dividing the area of the measured view by the number ofthe crystal grains to obtain the average area of the crystal grains, andcalculating a crystal grain size in an equivalent circular diameter. Atthat time, the grain on the boundary of the view was measured as ½, andan observation magnification ratio was adjusted if appropriate so thatthe number of the crystal grains was set to 200 or more.

<Number Density of Residual Carbides>

The surface of the hot-formed steel sheet member was corroded using apicral liquid, and magnified in a size of 2000 times with a scanningelectron microscope. A plurality of views were observed. At this time,the number of views in which carbides existed was counted to calculatethe number per 1 mm².

<Measurement of Local Deformability>

The local deformability was measured according to a notch tensile test.A tensile test piece had a parallel part width of 16.5 mm and a parallelpart length of 60 mm, and obtained with a rolling direction as alongitudinal direction. A 2-mm-deep V notch was processed in the lengthcentral part of the tensile test piece, and the processed tensile testpiece was used as a notch tensile test piece. The thickness of the notchtest piece was set to 1.4 mm. The shape of the notch tensile test pieceis shown in FIG. 3. The tensile test was performed using the notchtensile test piece, and notch elongation in a case in which the notchtensile test piece was broken at a V notched part was measured, toevaluate the local deformability. A reference point distance was set to5 mm, and a tensile speed (crosshead speed) during the tensile test wasset to 0.5 mm/min.

<Variation in Hardness>

The following test was performed in order to evaluate hardnessstability. Steel sheets for hot-forming were heated at 10° C./sec to900° C. by a heat treatment simulator, and then held for 150 sec. Then,the steel sheets for hot-forming were cooled at the cooling rates ofabout 80° C./sec and 10° C./sec to room temperature. Each of the sampleswas subjected to a Vickers hardness test at the ¼ position of the sheetthickness of the section. Hardness measurement was performed based onJIS Z 2244 (2009) at five points with a test force set to 9.8 N, and theaverage thereof was obtained. The average value of the hardnesses at thecooling rate of about 80° C./sec and the average value of the hardnessesat the cooling rate of 10° C./sec were defined as HS₈₀ and HS₁₀, and thedifference ΔHv thereof was used as the index of the hardness stability.

In order to evaluate the hardness stability and local deformability ofeach of the samples, the samples having ΔHv of 50 or less and notchelongation of 6% or more were determined to be favorable.

As shown in Table 2, the test number 2 had a steel compositionsatisfying the range of the embodiments, but the amount of molten steelto be cast per unit time was large. Thereby, the value of thecleanliness level exceeded 0.08%, which resulted in poor localdeformability.

Since the test number 3 was not subjected to a center segregationreducing treatment and a soaking treatment, the segregation degree of Mnexceeded 1.6, which resulted in poor local deformability.

Since the test number 5 had a low molten steel heating temperature, thevalue of the cleanliness level exceeded 0.08%, which resulted in poorlocal deformability.

Since the test number 6 had a low hot-forming temperature, the volumefraction of ferrite exceeded 3% after hot-forming, which resulted inpoor hardness stability. Furthermore, the number density of residualcarbides was also as high as 8.0×10³ per mm², which resulted in poorlocal deformability.

Since the test number 9 had a high heating temperature duringhot-forming, the prior γ grain size was increased, which resulted inpoor local deformability.

Since the test number 11 had a high winding temperature afterhot-rolling, the density of residual carbides was increased, whichresulted in poor local deformability.

Since the test number 14 had a high annealing temperature afterhot-rolling and a long annealing time, the volume fraction of ferriteexceeded 3% after hot-forming, which resulted in poor hardnessstability. The insufficient dissolution of carbides caused an increasein the density of residual carbides, which resulted in poor localdeformability.

Since the test number 16 had an S content exceeding the upper limitvalue of the range of the embodiments, the value of the cleanlinesslevel exceeded 0.08%, which resulted in poor local deformability.

Since the test number 17 had a Mn content exceeding the upper limitvalue of the range of the embodiments, the segregation degree of Mnexceeded 1.6, which resulted in poor local deformability.

Since the test number 18 had an Si content exceeding the upper limitvalue of the range of the embodiments, an A₃ point was increased, andthe volume fraction of ferrite exceeded 3% after hot-forming, whichresulted in poor hardness stability.

The test number 19 had a C content exceeding the upper limit value ofthe range of the embodiments, which resulted in poor localdeformability.

The test number 20 had a Cr content lower than the range of theembodiment, which resulted in poor hardness stability.

The test numbers 1, 4, 7, 8, 10, 12, 13, and 15 satisfying the range ofthe embodiments were excellent in both hardness stability and localdeformability.

The entire disclosures of Japan Patent Application No. 2014-101443 filedin May 15, 2014 and Japan Patent Application No. 2014-101444 filed inMay 15, 2014 are incorporated herein by reference.

All publications, patent applications, and technical standards describedherein are herein incorporated by reference to the same extent as ifeach individual publication, patent application, or technical standardwas specifically and individually indicated to be incorporated byreference.

As described above, the various typical embodiments have been described,but the invention is not limited to these embodiments. The range of theinvention is limited by only the following claims.

1. A hot-formed steel sheet member having a chemical compositionconsisting of, in terms of mass %, from 0.08 to 0.16% of C, 0.19% orless of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or lessof S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0to 0.50% of Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to1.0% of Mo, from 0 to 1.0% of V, from 0 to 0.005% of Ca, and a remainderconsisting of Fe and impurities, wherein a total volume fraction ofmartensite, tempered martensite, and bainite is 50% or more, and avolume fraction of ferrite is 3% or less, an average grain size of priorγ grains is 10 μm or less, and a number density of residual carbideswhich are present is 4×10³ per mm² or less.
 2. The hot-formed steelsheet member according to claim 1, wherein the chemical compositioncomprises one or more selected from the group consisting of, in terms ofmass %, from 0.003 to 0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to1.0% of Cu, from 0.01 to 1.0% of Mo, from 0.01 to 1.0% of V, and from0.001 to 0.005% of Ca.
 3. The hot-formed steel sheet member according toclaim 1, wherein a value of a cleanliness level of steel specified byJIS G 0555 (2003) is 0.08% or less.
 4. The hot-formed steel sheet memberaccording to claim 1, wherein a segregation degree α of Mn representedby the following formula (i) is 1.6 or less,α=[maximum Mn concentration (mass %) at a central part of a sheetthickness]/[average Mn concentration (mass %) at a ¼ depth position ofthe sheet thickness from a surface]  (i).
 5. The hot-formed steel sheetmember according to claim 1, wherein the steel sheet member has asurface on which a plating layer is formed.
 6. The hot-formed steelsheet member according to claim 1, wherein the steel sheet member hastensile strength of 1.0 GPa or more.
 7. A hot-formed steel sheet memberhaving a chemical composition comprising, in terms of mass %, from 0.08to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% orless of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% orless of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from0.001 to 0.01% of B, from 0 to 0.50% of Nb, from 0 to 2.0% of Ni, from 0to 1.0% of Cu, from 0 to 1.0% of Mo, from 0 to 1.0% of V, from 0 to0.005% of Ca, and a remainder comprising Fe and impurities, wherein atotal volume fraction of martensite, tempered martensite, and bainite is50% or more, and a volume fraction of ferrite is 3% or less, an averagegrain size of prior γ grains is 10 μm or less, and a number density ofresidual carbides which are present is 4×10³ per mm² or less.
 8. Thehot-formed steel sheet member according to claim 7, wherein the chemicalcomposition comprises one or more, in terms of mass %, from 0.003 to0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01to 1.0% of Mo, from 0.01 to 1.0% of V, or from 0.001 to 0.005% of Ca.